The present invention relates to a method for producing spherical particles of carbon or activated carbon with a uniform particle size.
More particularly, the present invention relates to a method for producing spherical particles of carbon or of activated carbon from a carbonaceous pitch wherein the pitch is mixed with a viscosity-reducing agent, the pitch and viscosity-reducing agent mixture (hereafter referred to as the mixture) is formed into spherical particles and then the viscosity-reducing agent is extracted from the spherical particles. The spherical particles of the pitch are then infusibilized and baked or activated to form the desired spherical particles of carbon or activated carbon. The invention is further characterized in that the formation of the spherical particles from the mixture is carried out by steps including melt-extruding the mixture through a nozzle to form string-like bodies, simultaneously cooling and stretching the string-like bodies by letting said string-like bodies flow together with water flowing at a rate of 0.5 to 5 m/sec on an inclined conduit or an inclined belt-conveyer. The thus formed mixture of brittle string-like bodies having diameters of 0.2 to 2.5 mm, are broken into stick-like bodies with a length to diameter ratio of 1 to 5, and then put into hot water, kept at a temperature higher than the softening point of the mixture. This step transforms the stick-like bodies into the spherical particles and upon cooling of the spherical particles in cold water, kept at a temperature lower than the softening point of the mixture, the solid spherical particles of the mixture are collected.
The spherical particles of carbon or activated carbon are utilized in treating industrial waste water, purifying drinking water, removing sulfur oxides from exhaust gases, etc. It has been known that their spherical shape contributes to their extreme effectiveness. Particularly effective are particles made from pitch; they are high in anti-collapsing strength, resistant to pulverization, and are used effectively in fixed and fluidized beds.
As a method for producing spherical particles of carbon or activated carbon from petroleum pitch or coal pitch, the method disclosed in Japanese Patent Publications No. 18879/75 and No. 76/76 is mentioned. According to the disclosed method, after adding a viscosity-reducing agent to a specified pitch and uniformly mixing them, the mixture is melted at 50.degree. to 300.degree. C. and dispersed into water containing a suspension agent at ordinary pressure or under pressure to form spherical particles of pitch. The thus formed pitch particles are treated with a solvent to extract the viscosity-reducing agent from the particles and then the particles are infusibilized and baked or activated.
Although the above-mentioned method is excellent for obtaining spherical particles of carbon or activated carbon, the size distribution of the thus obtained particles is very broad because the viscous fluid pitch mixture is cut and recombined in water while the mixture is still soft and sticky. Further, where the pitch is highly viscous, it is necessary to disperse the pitch mixture at a high temperature in order to sufficiently reduce the viscosity of the melted pitch. In such a condition it is difficult to obtain the particles of a small diameter because of the difficulty of breaking the larger particle with only shearing forces. The size distribution of the thus prepared particles ranges from small to large in diameter, and where the uniformly sized particles are desired, they would have to be sifted with sieves. Accordingly, a method of producing spherical particles of carbon or activated carbon with an uniform size has long been desired.
The inventors, after studying the methods for producing spherical particles of carbon or activated carbon have found that spherical particles of extremely uniform size may be prepared by a process comprising the steps of (1) melt-extruding a mixture of petroleum pitch or coal pitch and a viscosity-reducing agent through nozzles into a form of strings, (2) cooling and stretching the thus extruded strings to a solid state, (3) breaking the thus stretched and solidified strings into a stick form of a length to diameter ratio of less than 5, (4) putting the solidified sticks of the mixture into hot water at a temperature higher than the softening point of the mixture thus deforming the sticks into spherical particles by virtue of the surface tension properties of the mixture, (5) transferring the still-hot spherical particles into cold water to solidify the spherical particles, (6) collecting and separating the thus solidified spherical particles, (7) subjecting the spherical particles to extraction with a solvent to remove the viscosity-reducing agent therefrom, and (8) subjecting the thus obtained spherical particles of pitch to the conventional steps of infusibilization and baking or activation.
Accordingly, the present invention provides a method for producing spherical carbon or activated carbon of particles uniform size.
The present invention will be briefly explained while referring to the attached drawings and each step of the method of the present invention will be explained in more detail.